Conventionally, optical disk recording and reproducing devices use an optical pickup that incorporates a semiconductor laser (LD: Laser Diode) as a light source. One way to reduce noise in such an optical pickup is to switch the oscillation mode of the laser beam from a single mode to a multi mode mainly by high-frequency modulation (300 MHz to 900 MHz) of the laser beam.
With the recent advancement in high density optical disks (exceeding 10 GB), a blue laser of a short wavelength (400 nm) is beginning to be used for the optical pickup. However, the blue laser produces serious noise in a low power range at or below 2 mW, as shown in FIG. 2, and enough S/N ratio cannot be obtained by the conventional measure of reducing noise by high frequency modulation.
Further, writing information in an optical disk used as a recording medium requires a large laser power for the optical disk. In this case, the semiconductor laser in the optical pickup is used in a high power range, and the noise does not pose a serious problem. On the other hand, in reading information from the optical disk, the semiconductor laser in the optical pickup is used in a low power range, and accordingly the S/N ratio of the output laser beam is low. Further, because the reflected light of the optical disk returns to the semiconductor laser source, the returned light generates noise.
Such a problem is acknowledged in Japanese Publication for Unexamined Patent Application No. 131683/1994 (Tokukaihei 6-131683; published on May 13, 1994), and No. 150601/2002 (Tokukai 2002-150601; published on May 24, 2002), for example. These publications disclose disposing an ND (Neutral Density) filter in an optical path of the laser beam when reproducing information from the optical disk, so as to increase the output laser power in reproducing, and to attenuate the power of the returned light to reduce laser noise.
However, the conventional structure in which the ND filter is disposed in an optical path of the laser beam causes the following problems when it is used in a device operable to record and reproduce information for a single-layer disk and a bi-layer disk, or a device operable to reproduce information in two reproducing modes: a reproducing mode for reproducing information at a normal speed; and a reproducing mode for reproducing information at a double speed.
Reproducing information from a bi-layer disk or reproducing information at a double speed or faster speed requires relatively high reproducing power. Thus, when reproducing information from a bi-layer disk or when reproducing information at a double speed or faster speed (“high power reproducing” hereinafter), the laser noise of the laser diode does not always cause a problem even when the light efficiency of the laser diode is not attenuated using the ND filter.
However, when the ND filter is disposed in an optical path of the laser beam to reproduce information from a single-layer disk or to reproduce information at a normal speed, the output of the laser diode is lost in excess in high power reproducing, with the result that power consumption is increased.
Further, in high power reproducing, when the output of the laser diode is increased further to compensate for the loss through the ND filter, the temperature of the laser diode increases, and the life of the laser diode becomes shorter.
For the optimum control of light efficiency of the laser diode, the ND filter may be removed from the optical path of the laser beam in high power recording. However, this requires a mechanism for inserting and removing the ND filter. In addition, it takes time to insert or remove the filter. That is, it takes time to switch operations requiring the ND filter and not requiring the ND filter.